Human evolution

Human evolution is the part of the theory of evolution by which human beings emerged
as a distinct species. It is the subject of a
broad scientific
inquiry that seeks to understand and describe how this change and
development occurred. The study of human evolution encompasses many scientific
disciplines, most notably physical
anthropology and genetics. The term 'human', in
the context of human evolution, refers to the genus Homo, but studies
of human evolution usually include other hominins, such as the australopithecines.

History of paleoanthropology

The modern field of paleoanthropology
began with the discovery of 'Neanderthal man'; and
evidence of other 'cave men' in the 19th
century. The idea that humans are similar to certain great apes had been obvious to
people for some time, but the idea of the biological evolution of species in
general was not legitimized until after Charles Darwin published
On the Origin of
Species in 1859. Though Darwin's first book on
evolution did not address the specific question of human evolution— "light will
be thrown on the origin of man and his history," was all Darwin wrote on the
subject— the implications of evolutionary theory were clear to contemporary
readers. Debates between Thomas Huxley and Richard
Owen focused on the idea of human evolution, and by the time Darwin
published his own book on the subject, Descent of Man, it
was already a well-known interpretation of his theory— and the interpretation
which made the theory highly controversial. Darwin believed that Caucasians
evolved from chimpanzees, that Africans evolved from the less intelligent yet
stronger apes, and that Asians evolved from orangutangs. Even many of Darwin's
original supporters (such as Alfred Russel
Wallace and Charles Lyell) balked at
the idea that human beings could have evolved their apparently boundless mental
capacities and moral sensibilities through natural selection.

Since the time of Carolus Linnaeus, the
great apes were considered the closest relatives of human beings, based on
morphological similarity. In the 19th century, it was speculated that our
closest living relatives were chimpanzees and gorillas, and
based on the natural range of these creatures, it was surmised humans share a common ancestor with African apes and
that fossils of these ancestors would ultimately be found in Africa.

It was not until the 1920s that fossils other than
neanderthalensis were discovered. In 1924, Raymond Dart described
Australopithecus
africanus. The type specimen was the Taung
Child, an australopithecine
infant discovered in Taung, South Africa. The remains
were a remarkably well-preserved tiny skull and an endocranial cast of the
individual's brain. Although the brain was small (410 cm³), its shape was
rounded, unlike that of chimpanzees and gorillas, and more like a modern human
brain. Also, the specimen exhibited short canine teeth, and the
position of the foramen magnum was
evidence of bipedal locomotion. All of these
traits convinced Dart that the Taung baby was a bipedal human ancestor, a
transitional form between apes and humans. Another 20 years would pass before
Dart's claims were taken seriously, following the discovery of more fossils that
resembled his find. The prevailing view of the time was that a large brain
evolved before bipedality. It was thought that intelligence on par with modern
humans was a prerequisite to bipedalism.

The australopithecines are now thought to be the immediate ancestors of the
genus Homo, the group to which modern humans belong. Both
australopithecines and Homo sapiens are part of the tribe Hominini, but
recent data has brought into doubt the position of A. africanus as a
direct ancestor of modern humans; it may well have been a dead-end cousin. The
australopithecines were originally classified as either gracile or robust. The robust variety of
Australopithecus has since been reclassified as Paranthropus. In the 1930s, when the robust
specimens were first described, the Paranthropus genus was used. During
the 1960s, the
robust variety was moved into Australopithecus. The recent trend has been
back to the original classification as a separate genus.

The Homo genus

In modern taxonomy, Homo sapiens is the only extant species of its genus, Homo. Likewise, the
ongoing study of the origins of Homo sapiens often demonstrates that
there were other Homo species, all of which are now extinct. While some
of these other species might have been ancestors of H. sapiens, many were
likely our 'cousins', having speciated away from our ancestral line. There is
not yet a consensus as to which of these groups should count as separate species
and which as subspecies of another species. In some cases this is due to the
paucity of fossils, in other cases it is due to the slight differences used to
classify species in the Homo genus.

The word homo is Latin for 'person', chosen
originally by Carolus Linnaeus in his
classification system. It is often translated as 'man', although this can lead
to confusion, given that the English word 'man' can be generic like homo,
but can also specifically refer to males. Latin for 'man' in the gender-specific
sense is vir, cognate with "virile" and
"werewolf". The word 'human' is from humanus, the adjectival form
of homo.

Homo habilis

H. habilis lived from
about 2.4 to 1.5 million years ago (MYA). H. habilis, the first species
of the genus Homo, evolved in South and East Africa in the late Pliocene or
early Pleistocene, 2.5–2 MYA, when
it diverged from the Australopithecines. H. habilis had smaller molars and larger brains than the
Australopithecines, and made tools from stone and perhaps
animal bones. One
of the first known hominids, it was nicknamed 'handy man' by its discoverer, Louis
Leakey.

Homo erectus

H. erectus (including
H.
ergaster) lived from about 1.8 MYA (or from about 1.25 MYA excluding
ergaster) to 0.07 MYA. In the Early Pleistocene, 1.5–1 MYA, in Africa, Asia, and Europe, presumably,
Homo
habilis evolved larger brains and made more elaborate stone tools; these
differences and others are sufficient for anthropologists to classify them as a
new species, H. erectus. In addition
H. erectus was the first human ancestor to walk truly upright. This was
made possible by the evolution of locking knees and a different location of the
foramen magnum (the hole
in the skull where the spine enters). A famous example of Homo erectus is
Peking
Man; others were found in Asia (notably in Indonesia), Africa, and Europe.
Many paleoanthropologists are now using the term Homo ergaster for the
non-Asian forms of this group, and reserving H. erectus only for those
fossils found in the Asian region and meeting certain skeletal and dental
requirements which differ slightly from ergaster. They may have used fire to cook their meat.

Homo ergaster

Homo heidelbergensis

H.
heidelbergensis (Heidelberg Man) lived from
about 800 thousand years ago (TYA) to about 300 TYA. Also proposed as Homo
sapiens heidelbergensis and Homo sapiens paleohungaricus.

Homo sapiens idaltu

H. sapiens
idaltu lived from about 160 TYA (proposed subspecies). It is the oldest
known anatomically modern human.

Homo floresiensis

H. floresiensis,
which lived to about 12 TYA (announced 28 October2004 in the science journal Nature), has
been nicknamed hobbit for its small size,
probably a result of Island dwarfing. H.
floresiensis is intriguing both for its size and its age, being by far the
most recent species of Homo that does not lie along the direct evolutionary path
of modern humans. Found in 2003 it has been dated to approximatly 18,000 years
old. The main find was a fossil believed to be a woman of about 30 years of age.
Her brain size was only 380cc (which can be considered small even for a chimp).
She was only 1 meter in height.

Homo neanderthalensis

H.
neanderthalensis lived from about 250 to 30 TYA. Also proposed as
Homo sapiens neanderthalensis. There is ongoing debate over whether the
'Neanderthal Man' was a
separate species, Homo neanderthalensis, or a subspecies of H.
sapiens. While the debate remains unsettled, the prevailing view of
evidence, collected by examining mitochondrial DNA and
Y-chromosomalDNA, currently indicates that little
or no gene flow occurred between H. neanderthalensis and H.
sapiens, and, therefore, the two were separate species. In 1997, Dr. Mark Stoneking, then an
associate professor of anthropology at Pennsylvania
State University, stated: "These results [based on mitochondrial DNA extracted
from Neanderthal bone] indicate that Neanderthals did not contribute
mitochondrial DNA to modern humans… Neanderthals are not our ancestors."²
Subsequent investigation of a second source of Neanderthal DNA confirmed these
findings.³ However, supporters of the multiregional
hypothesis point to recent studies indicating non-African nuclear DNA
heritage dating to one MYA, as well as apparent hybrid fossils found in Portugal and
elsewhere, in rebuttal to the prevailing view.

Homo sapiens

H.
sapiens has lived from about 200 TYA to the present. Between 400,000
years ago and the second interglacial period in the Middle Pleistocene, around 250,000
years ago, the trend in cranial expansion and
the elaboration of stone tool technologies developed, providing evidence for a
transition from H. erectus to H. sapiens. The direct
evidence suggests there was a migration of H.
erectus out of Africa, then a further speciation of H. sapiens from
H. erectus in Africa (there is little evidence that this speciation
occurred elsewhere). Then a subsequent
migration within and out of Africa eventually replaced the earlier dispersed
H. erectus. However, the current evidence does not preclude
multiregional speciation, either. This is a hotly debated area in paleoanthropology.
Sapiens means wise or intelligent. Current research establishes that human
beings are highly genetically homogenous, meaning that the DNA of individual
Homo Sapiens is more alike than usual for most species, a result of our
relatively recent evolution. Distinctive genetic characteristics have arisen
however, primarily as the result of small groups of people moving into new
environmental circumstances. Such small groups are initially highly inbred,
allowing the relatively rapid transmission of traits favorable to the new
environment. These adapted traits are a very small component of the Homo Sapiens
genome and include such outward "racial" characteristics as skin color and nose
form in addition to internal characteristics such as the ability to breathe more
efficiently in high altitudes.